This invention relates to a transducer formed as a surface acoustic wave device which has better transduction characteristics for the same length transducer thus allowing a more compact device to be constructed, has a higher input conductance for the same length transducer and has a more symmetrical conductance thus providing less distortion to input signals applied thereto. It has particular utility in a resonator and a resonator-filter.
When conventional types of SAW transducers with two electrodes per wavelength are used as impedance elements, the conductance is maximum at a frequency offset from the impedance element design frequency because of the acoustic wave reflections occurring within the device. Thus, the conductance of the device at the design frequency is decreased because of the electrical reflections within the element. More importantly the peak of maximum conductance then is shifted from the design frequency.
Prior art two-port resonators such as those disclosed in U.S. Pat. No. 4,144,507 to Shreve and U.S. Pat. No. 4,454,488 to Hartmann also use SAW transducer elements having several spurious reponse peaks of different amplitude with the largest spurious peak at or near the frequency at which the resonator is desired to operate. However, under certain conditions it is possible that the device may lock onto the frequency exhibited at the highest amplitude adjacent peak. Of course, this is disadvantageous inasmuch as the resonator exhibits unstable operating conditions. Further, there may be a slight decrease in Q exhibited by the device due to the high amplitude adjacent peak.
SAW devices find large use in resonator filters because there are size and cost advantages which lead to replacing conventional LC bandpass filters. These resonator filters typically involve a piezoelectric substrate with interdigital input and output transducers formed on the substrate.
Although the frequency response has a major peak at the frequency at which it is desired that the resonator filter operate, the frequency response also shows several other spaced but lesser amplitude peaks at other frequencies close to the operating frequency. This, of course, allows unwanted frequencies to pass through the filter. To suppress the unwanted peaks, special steps must be taken such as using grooves or changing the frequency of the transducers with respect to the gratings which complicates both the construction and the operation of the resonator filter. See U.S. Pat. No. 4,325,037.
A unique two port resonator can be formed using a modified transducer element as the input transducer, a standard prior art transducer element as the output transducer and standard gratings used in conjunction with the input and output transducers. It has all of the advantages of existing two port resonator structures as shown and described in U.S. Pat. No. 4,144,507 to Shreve and U.S. Pat. No. 4,454,488 to Hartmann and, in addition, has essentially one major transmission peak at the design frequency instead of several. While there are other adjacent peaks, they are, to a great extent suppressed below the one major peak thus preventing an oscillator formed from the resonator from locking on to the frequency of the lower adjacent peaks as can happen with the prior art oscillators. In addition, the Q of the novel input transducer is slightly better than the prior art transducers.
The resonator and resonator filter of the present invention utilizes a modified input transducer and an output transducer of normal design. The novel input transducer provides an admittance response that more nearly peaks at the design frequency of the transducer and has a null in its input conductance which can be adjusted by withdrawal weighting, or other types of weighting, to occur at or near any desired frequency. However, the output transducer has a peak in the conductance, depending upon the substrate material and overlay, which will occur offset from the design frequency. By adjusting the null of the input transducer to coincide with the conductance peak of the output transducer, spurious responses of the resonator or resonator filter may be suppressed. Thus, a good net frequency response that is far improved over prior art responses is obtained. These results are obtained with .lambda./4 electrodes with no recessed or split finger electrodes, and without grooves or frequency shifts between the transducers and the gratings thereby allowing such filters to be designed at much higher frequencies and with a greater economy in construction.